The present invention relates to a process for the qualitative and quantitative measurement of irregularities and impurities on and in transparent or semitransparent flexible sheet materials, in particular, film webs. The invention also pertains to a device for carrying out the process.
In the process, interference rings are produced by means of a light beam directed at the surface of the sheet, part of the light beam being reflected from the surface of the sheet and part of it being reflected from another surface after passing through the sheet. The two partial beams are superposed again and fed to a recording device. Apart from this process, polarization effects are created in the sheet by means of a penetrating polarized light beam, which is also fed to a recording device.
Flexible materials, in particular in the form of polymer films, are used in many technical fields for a great number of different purposes. For some applications, such as, e.g., magnetic tape films or capacitor films, it is necessary to subject the surface of the base film to continuous quality control tests. In cases where very thin films are coated, for example, with magnetic coatings to give audiotapes or videotapes, or where films of this type are used for producing roll-type capacitors, it is indispensible to know the surface quality in order to obtain usable end products.
For example, it is known in magnetic tape production to control the surface quality before and after coating, since variations in the distance between the magnetic head and the surface of the magnetic tape affect both the recording quality and the voltage level read. As a consequence, therefore, the surface quality must be continuously controlled.
One frequently employed control method is the use of an interference microscope, where the enlarged image of the surface to be examined is superposed by interference phenomena. The interference phenomena in the form of interference rings or fringes are created by splitting the light beams of a monochromatic light source into two partial bundles of beams, by surface reflection and diffraction or reflection from a mirror, and superposing these partial bundles of beams again. As a result, parallel interference lines having a distance of half a wavelength appear in the microscope. In the case of sodium light, this distance is 0.29 .mu.m. If irregularities are present on the surface to be controlled, deflection of the interference lines occurs, whereby the height of these irregularities can be determined with the aid of the line distance or the number of intereference rings surrounding the irregularities.
When examining the surfaces of magnetic tapes, it has been found that the irregularities, i.e., elevations or indentations, have diameters which rarely exceed 25 .mu.m, and in some cases are even less than 2 .mu.m. In most cases, they have the shape of points, arches or cones. In order to be able to successfully employ this known process for determining the surface quality, highly magnifying microscopes (in general 250-power or 500-power) must be used, which means that the area which can be examined is not larger than 0.01 mm.sup.2 to 0.05 mm.sup.2. This is not adequate, however, for an appropriate and reliable control of the surface quality.
German Offenlegungsschrift No. 19 20 928 discloses a device for examining the evenness and smoothness of surfaces by means of interference measurement, whereby the surface to be examined is exposed to light and then inspected obliquely through a planar surface of glass or the like which is adjacent, but not in direct contact with the surface to be inspected, the angles of exposure and inspection being chosen such that no total reflection is caused.
European Patent Application No. 00 32 710 is concerned with a process and device for analysis of the surface properties of flexible materials, in particular of film webs, by producing interference fringes by means of a light beam directed obliquely onto the surface to be examined, whereby a portion of the light beam is reflected from another surface, and the two partial beams are superposed again. The object to be examined is put on a reflecting reference surface and the partial beams are formed by reflection from the reference surface and the surface to be examined so that an interference field is obtained which is equivalent to the surface of the object to be measured, i.e., which shows the differences as compared with the profile of the reference surface. This interference field is then evaluated in a known manner to determine the extension of the surface profile in the horizontal and vertical directions, with the aid of the distances between the interference lines and the wavelength of the irradiated light.
Although the two last mentioned processes bring about essential improvements in the process described initially, they nevertheless still have a serious disadvantage. Thus, it is possible to determine the number and height of superficial irregularities using these methods, but it is not possible to distinguish between irregularities which are due to particles (for example, dust) adhering to the surface, and irregularities which are due to particles embedded between the surfaces (for example, blisters or specks).
Such a differentiation is, however, of crucial importance for a manufacturer of flexible sheet materials which are, e.g., to be used as audio or videotapes or as capacitor films. Only if the process operator knows the exact nature of the irregularities, can he decide whether it will be necessary to change the process conditions in respect of avoiding abrasion, electric charging during the film transport (attraction of dust particles), etc., whether the formulation of the raw materials or the manufacturing conditions will have to be changed or whether, for example, other filters and service lives of the filters will have to be chosen.